In typical surface-mount circuit board manufacturing operations, a stencil printer is used to print solder paste onto a circuit board. Typically, a circuit board having a pattern of pads or some other conductive surface onto which solder paste will be deposited is automatically fed into the stencil printer and one or more small holes or marks on the circuit board, called fiducials, is used to properly align the circuit board with a stencil or screen of the printer prior to the printing of solder paste onto the circuit board. After the circuit board is aligned, the board is raised to the stencil, solder paste is dispensed onto the stencil, and a wiper blade (or squeegee) traverses the stencil to force the solder paste through apertures formed in the stencil and onto the board.
In some prior art stencil printers, a dispensing head delivers solder paste between first and second wiper blades, wherein the one of the wiper blades is used to move or roll solder paste across the stencil in a direction opposite to the direction of the other wiper blade. The first and second wiper blades are used on alternating boards to continually pass the roll of solder paste over the apertures of a stencil to print each successive circuit board. The wiper blades are typically at a predetermined angle with the stencil to apply downward pressure on the solder paste to force the solder paste through the apertures of the stencil.
This type of dispensing system is illustrated in FIGS. 1-4. Specifically, a dispensing head 10 deposits a bead 12 of solder paste on a stencil 14. This step is illustrated in schematic form in FIG. 1. Next, wiper blades 16, 18 are positioned over the bead 12 of solder paste in such a manner that the leading blade 18 is lifted off of the stencil 14 and the trailing blade 16 engages the stencil (see FIG. 2). During a print stroke, the trailing blade 16 rolls the solder paste to force the solder paste into the apertures (not shown) of the stencil 14. The force applied by the wiper blade 16 forces the solder paste into the apertures provided in the stencil 14. Upon completing the print stroke, the leading blade 18 shown in FIGS. 2 and 3 is lowered to engage the stencil 14 and the trailing blade 16 (as shown in FIGS. 2 and 3) is lifted off of the stencil. With reference to FIG. 4, blade 16 becomes the leading blade and blade 18 becomes the trailing blade that rolls the solder paste across the stencil 14 during a subsequent print operation.
One disadvantage of the system disclosed in FIGS. 1-4 is that the wiper blades have to be manipulated, e.g., by lifting one blade so that solder paste can be dispensed between the wiper blades or by moving the entire wiper blade assembly as illustrated in FIG. 1, to replenish the solder paste.
Another approach to printing solder paste can be found in U.S. Pat. No. 5,947,022, issued on Sep. 7, 1999 to Freeman et al. In one system described in this patent, a dispensing head having two wiper blades is pressurized by applying air pressure to the solder paste being dispensed. Specifically, when the dispensing head is in a lowered printing position, solder paste is forced from the dispensing head onto the stencil using air pressure as the dispensing head is moved across the stencil. For each direction that the dispensing head moves across the stencil, one of the wiper blades will be a trailing blade and will scrape any excess solder paste off the stencil. In the system described in U.S. Pat. No. 5,947,022, pressure provided by the dispensing head, rather than its wiper blades, forces solder paste through the stencil. At the conclusion of printing, when it is a desired to lift the dispensing head off of the stencil, the pressurized air is turned off prior to lifting the dispensing head off of the stencil. It should be understood that the pressure of the solder paste is carefully maintained in a dispensing chamber of the dispensing head to achieve effective printing operations.